There are many references in the literature to prostaglandins or prostanoids (PGs), a term which is generic to natural and synthetic prostaglandins and prostaglandin-like compounds, and it is well known that even slight differences in their chemical structures or stereochemical configurations will have profound effects on their biological activity.
Prostaglandins or prostanoids (PGs) are a group of bioactive compounds derived from membrane phospholipids, and are formed from 20-carbon essential fatty acids and a cyclopentane ring. They fall into several main classes designated by letters and are distinguished by substitutions to the cyclopentane ring. The main classes are further subdivided by subscripts 1, 2, or 3 which reflect their fatty acid precursors.
An example of a particular species of the prostaglandin E is PGE2, with the following structure: At present four different receptor subtypes of PGE2 receptors are known and they are designated EP1, EP2, EP3, and EP4. 
Uses for compounds possessing binding activity similar to PGE2 comprise the prevention and/or treatment of immunological diseases (autoimmune diseases, organ transplantation, etc.), asthma, abnormal bone formation, neuronal cell death, thrombosis and stroke, hepatopathy, abortion, male and female sexual dysfunction, premature birth, inflammation such as rheumatoid arthritis, or retina neuropathy disorders such as glaucoma.
Prostaglandins and their associated receptors are more fully described in for example: M. Abramovitz et al., The Utilization of Recombinant Prostanoid Receptors to Determine the Affinities and Selectivities of Prostaglandins and Related Analogs, Biochimica et Biophysica Acta 2000, 1483, 285-293.
The involvement of prostaglandin E receptor agonists in bone resorption is described in, e.g., T. Suzawa et al., The Role of Prostaglandin E Receptor Subtypes in Bone Resorption: An Analysis Using Specific Agonists for the Respective EPs, Endocrinology 2000, 141, 1554-1559; K. Ono et al., Important Role of EP4, a Subtype of Prostaglandin (PG) E Receptor, in Osteoclast-like Cell Formation from Mouse Bone Marrow Cells Induced by PGE2, J. of Endocrinology 1998, 158, R1-R5; M. Suda et al., Prostaglandin E Receptor Subtypes in Mouse Osteoblastic Cell Line, Endocrinology 1996, 137, 1698-1705.
These selective prostaglandin E receptor agonists are also useful for the treatment of gastric lesions, see e.g. H. Araki, et al. The Roles of Prostaglandin E Receptor Subtypes in the Cytoprotective Action of Prostaglandin E2 in Rat Stomach, Aliment. Pharmacol. Ther. 2000, 14 (Suppl. 1), 116-124; T. Kunikata, et al., E Type Prostaglandin Inhibits Indomethacin-Induced Small Intestinal Lesions Through EP3 and EP4 Receptors: A Study Using Rats and Knockout Mice, Gastroenterology 118, abstract #3787.
Other uses of prostaglandin E receptor agonists are for improvement of kidney function as described in, e.g., M. D. Breyer, et al, Prostaglandin E Receptors and the Kidney, Am. J. Physiol. 2000, 279, F12-F23, and K. E. Purdy, et al., EP1 and EP4 Receptors Mediate Prostaglandin E2 Actions in the Microcirculation of Rat Kidney, Am. J. Physiol. 2000, 279, F755-F764; for thrombosis and stroke as well as for other conditions where an inhibition of platelet aggregation would be beneficial as described in, e.g., B. Z. S. Paul, et al, Distribution of Prostaglandin IP and EP Receptor Subtypes and Isoforms in Platelets and Human Umbilical Artery Smooth Muscle Cells, Br. J. Haematol. 1998, 102, 1204-1211; for antiinflammatory effects through inhibition of TNF-alpha generation as described in, e.g. K. K. Meja, et al. Characterization of prostanoid receptor(s) on human blood monocytes at which prostaglandin E2 inhibits lipopolysaccharide-induced tumor necrosis factor-alpha generation, Br. J. Pharmacol. 1997, 122, 149-157, and A. Eigler, et al. Anti-inflammatory activities of cAMP-elevating agents: enhancement of IL-10 synthesis and concurrent suppression of TNF production, J. Leukoc. Biol. 1998, 63, 101-107; or for glaucoma as described in, e.g., M. Takamatsu, et al. Localization of Prostaglandin E Receptor Subtypes in The Ciliary Body of Mouse Eye, Exp. Eye Res. 2000, 70, 623-628, and D. F. Woodward, et al, Molecular Characterization and Ocular Hypotensive Properties of the Prostanoid EP2 Receptor, J. Ocul. Pharmacol. Ther. 1995, 11, 447.
Treatment of impotence and/or erectile dysfunction by using prostaglandins that are selective EP2 and/or EP4 receptor agonists have been disclosed in International Application Publication No. WO 99/02164 assigned to Pharmacia & Upjohn AB.
Additional information relating to prostaglandins and their receptors is described in Goodman & Gillman's, The Pharmacological Basis of Therapeutics, ninth edition, McGraw-Hill, New York, 1996, Chapter 26, pages 601-616.
8-Aza-11-deoxy-prostaglandin analogs corresponding to PGE2 would have the following structure: 
Substitution of a nitrogen for the carbon at C-8 causes a change in the three dimensional conformation of the resultant prostaglandin, and because structure is related to biological activity, such a conformational change will have a profound effect upon the biological activity. 8-Aza-11-deoxy prostaglandin E analogues with the natural side chains have been reported in the literature, see e.g. BE 841,165, assigned to Syntex USA, Inc.
Compounds of this invention are 8-azaprostaglandins with a non-natural side chain on the C-12 position of the pyrrolidone ring (following prostaglandin nomenclature), said chain containing a heterocyclyl, an aryl or a heteroaryl ring at the C-15 position of the chain. These compounds have high selectivity in their EP4 receptor agonist activity. The increase in selectivity would alleviate the severe side effects frequently observed following administration of non-selective prostaglandins agonists. Therefore compounds of this invention are desirable.